Finely ground coals in aqueous suspension are viable alternates for stoker coal, pulverized coal, No. 6 fuel oil, petroleum-based fuels, and natural gas. State-of-the art slurries of that character are described in U.S. patent application No. 406,730 filed June 10, 1982, and assigned to the assignee of the inventions described and claimed herein (now U.S. Pat. No. 4,515,602 dated May 7, 1985). Those coal-water slurries have a particle size distribution of at least 95% 30 m.times.0. The stirred ball mills disclosed and claimed herein are capable of wet grinding the solid particles in slurries containing up to 65 weight percent solids (or an even higher concentration of solids) to a size consist of that character at an acceptable operating cost and with an also acceptable capital investment.